CoFe/Ni Multilayer film with perpendicular anisotropy for microwave assisted magnetic recording

ABSTRACT

A spin transfer oscillator with a seed/SIL/spacer/FGL/capping configuration is disclosed with a composite seed layer made of Ta and a metal layer having a fcc(111) or hcp(001) texture to enhance perpendicular magnetic anisotropy (PMA) in an overlying (A1/A2) X  laminated spin injection layer (SIL). Field generation layer (FGL) is made of a high Bs material such FeCo. Alternatively, the STO has a seed/FGL/spacer/SIL/capping configuration. The SIL may include a FeCo layer that is exchanged coupled with the (A1/A2) X  laminate (x is 5 to 50) to improve robustness. The FGL may include an (A1/A2) Y  laminate (y=5 to 30) exchange coupled with the high Bs layer to enable easier oscillations. A1 may be one of Co, CoFe, or CoFeR where R is a metal, and A2 is one of Ni, NiCo, or NiFe. The STO may be formed between a main pole and trailing shield in a write head.

This is a Divisional application of U.S. patent application Ser. No.12/800,196, filed on May 11, 2010, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

RELATED PATENTS

This application is related to U.S. Pat. No. 8,064,244 and U.S. Pat. No.8,184,411; assigned to the same assignee and herein incorporated byreference in their entirety.

FIELD OF THE INVENTION

The invention relates to a high performance spin valve in which(CoFe/Ni)n multilayer structures having high perpendicular magneticanisotropy (PMA) are used to establish partial PMA in high momentmaterials such as FeCo through exchange coupling thereby enabling easierflux generation layer (FGL) oscillations in microwave assisted magneticrecording (MAMR).

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with MTJ technology, is a major emerging technology thatis highly competitive with existing semiconductor memories such as SRAM,DRAM, and Flash. Similarly, spin-transfer (spin torque or STT)magnetization switching described by C. Slonczewski in “Current drivenexcitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7(1996), has recently stimulated considerable interest due to itspotential application for spintronic devices such as STT-MRAM on agigabit scale. Recently, J-G. Zhu et al. described another spintronicdevice called a spin transfer oscillator in “Microwave Assisted MagneticRecording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008)where a spin transfer momentum effect is relied upon to enable recordingat a head field significantly below the medium coercivity in aperpendicular recording geometry.

Both MRAM and STT-MRAM have a MTJ element based on a tunnelingmagneto-resistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thinnon-magnetic dielectric layer, or based on a GMR effect where areference layer and free layer are separated by a metal spacer. The MTJelement is typically formed between a bottom electrode such as a firstconductive line and a top electrode which is a second conductive line atlocations where the top electrode crosses over the bottom electrode. AMTJ stack of layers may have a bottom spin valve configuration in whicha seed layer, an anti-ferromagnetic (AFM) pinning layer, a ferromagnetic“pinned” layer, a thin tunnel barrier layer, a ferromagnetic “free”layer, and a capping layer are sequentially formed on a bottomelectrode. The AFM layer holds the magnetic moment of the pinned layerin a fixed direction. The pinned or reference layer has a magneticmoment that is fixed in the “y” direction, for example, by exchangecoupling with the adjacent AFM layer that is also magnetized in the “y”direction. The free layer has a magnetic moment that is either parallelor anti-parallel to the magnetic moment in the pinned layer. The tunnelbarrier layer is thin enough that a current through it can beestablished by quantum mechanical tunneling of conduction electrons. Themagnetic moment of the free layer may change in response to externalmagnetic fields and it is the relative orientation of the magneticmoments between the free and pinned layers that determines the tunnelingcurrent and therefore the resistance of the tunneling junction. When asense current is passed from the top electrode to the bottom electrodein a direction perpendicular to the MTJ layers (CPP mode), a lowerresistance is detected when the magnetization directions of the free andpinned layers are in a parallel state (“1” memory state) and a higherresistance is noted when they are in an anti-parallel state or “0”memory state.

As the size of MRAM cells decreases, the use of external magnetic fieldsgenerated by current carrying lines to switch the magnetic momentdirection becomes problematic. One of the keys to manufacturability ofultra-high density MRAMs is to provide a robust magnetic switchingmargin by eliminating the half-select disturb issue. For this reason, anew type of device called a spin transfer (spin torque) device wasdeveloped. Compared with conventional MRAM, spin-transfer torque orSTT-MRAM has an advantage in avoiding the half select problem andwriting disturbance between adjacent cells. The spin-transfer effectarises from the spin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a CPP configuration, thespin angular moment of electrons incident on a ferromagnetic layerinteracts with magnetic moments of the ferromagnetic layer near theinterface between the ferromagnetic and non-magnetic spacer. Throughthis interaction, the electrons transfer a portion of their angularmomentum to the ferromagnetic layer. As a result, spin-polarized currentcan switch the magnetization direction of the ferromagnetic layer if thecurrent density is sufficiently high, and if the dimensions of themultilayer are small. The difference between a STT-MRAM and aconventional MRAM is only in the write operation mechanism. The readmechanism is the same.

Materials with PMA are of particular importance for magnetic andmagnetic-optic recording applications. Spintronic devices withperpendicular magnetic anisotropy have an advantage over MRAM devicesbased on in-plane anisotropy in that they can satisfy the thermalstability requirement but also have no limit of cell aspect ratio. As aresult, spin valve structures based on PMA are capable of scaling forhigher packing density which is a key challenge for future MRAMapplications and other spintronic devices.

Materials exhibiting PMA such as CoPt, CoPt—SiO₂, Tb(Fe)Co, and FePthave been reported multiple times in publications. However, all of theliterature examples suffer from at least one drawback. It is preferredthat establishing a PMA property in a spin valve structure does notrequire strenuous heating. Unfortunately, FePt or Tb(Fe)Co need hightemperature annealing to achieve high enough PMA which is unacceptablefor device integration since certain components are damaged by hightemperatures. CoPt and its alloys such as CoCrPt and CoPt—SiO₂ are notdesirable because Pt and Cr are severe spin depolarizing materials andwill seriously quench the amplitude of spintronic devices ifincorporated in the spinvalve structures. That leaves the novel magneticmultilayer systems such as Co/X where X=Pt, Pd, Au, Ni, Ir, and the likefor consideration. As stated above, Co/Pt, Co/Pd, and Co/Ir will not begood PMA materials for spintronic devices because of the severe spindepolarizing property of Pt, Pd, and Ir. Furthermore, Co/Pt, Co/Pd, andCo/Ir configurations typically require a very thick and expensive Pt,Pd, or Ir as a seed layer. Au is associated with high cost and easyinterdiffusion to adjacent layers which makes a Co/Au multilayer for PMApurposes less practical. On the other hand, a Co/Ni multilayerconfiguration as a PMA material candidate has several advantagesincluding (a) much higher spin polarization from Co, Ni, and Co/Niinterfaces, (b) better stability from the robustness of Ni layerinsertion, (c) much higher saturation magnetization of 1 Tesla or about2× higher than other Co/M combinations (M=metal), and (d) low cost.

PMA materials have been considered for MAMR applications as described byJ-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans.on Magn., Vol. 44, No. 1, pp. 125-131 (2008). A mechanism is proposedfor recording at a head field significantly below the medium coercivityin a perpendicular recording geometry. FIG. 1 is taken from theaforementioned reference and shows an ac field assisted perpendicularhead design. The upper caption 19 represents a perpendicular spin torquedriven oscillator for generating a localized ac field in a microwavefrequency regime and includes a bottom electrode 11 a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer orSIL), metallic spacer 13, and oscillating stack 14. Oscillator stack 14is made of a field generation layer (FGL) 14 a and a layer withperpendicular anisotropy 14 b having an easy axis 14 c. The ac fieldgenerator in the upper caption 19 is rotated 90 degrees with respect tothe lower part of the drawing where the device is positioned between awrite pole 17 and a trailing shield 18. The writer moves across thesurface of a magnetic media 16 that has a soft underlayer 15. Thereference layer 12 provides for spin polarization of injected current(I). Layers 14 a, 14 b are ferromagnetically exchanged coupled. Improvedmaterials for the reference layer and oscillator stack are needed asthis technology matures.

Several attempts disclosed in the literature have been made in order toachieve high PMA from Co/Ni multilayer configurations. However, all ofthe examples typically involve a very thick underlayer to establish PMA.For instance, G. Daalderop et al. in “Prediction and Confirmation ofPerpendicular Magnetic Anisotropy in Co/Ni Multilayers”, Phys. Rev.Lett. 68, 682 (1992) and F. den Broeder et al. in “Co/Ni multilayerswith perpendicular magnetic anisotropy: Kerr effect and thermomagneticwriting”, Appl. Phys. Lett. 61, 1648 (1992), use a 2000 Angstrom thickAu seed layer. In V. Naik et al., “Effect of (111) texture on theperpendicular magnetic anisotropy of Co/Ni multilayers”, J. Appl. Phys.84, 3273 (1998), and in Y. Zhang et al., “Magnetic and magneto-opticproperties of sputtered Co/Ni multilayers”, J. Appl. Phys. 75, 6495(1994), a 500 Angstrom Au/500 Angstrom Ag composite seed layer isemployed. Jaeyong Lee et al. in “Perpendicular magnetic anisotropy ofthe epitaxial fcc Co/60-Angstrom-Ni/Cu(001) system”, Phys. Rev. B 57,RS728 (1997) describe a 1000 Angstrom thick Cu seed layer. A 500Angstrom Ti or 500 Angstrom Cu seed layer with heating to 150° C. isused by P. Bloemen et al. in “Magnetic anisotropies in Co/Ni (111)multilayers”, J. Appl. Phys. 72, 4840 (1992). W. Chen et al. in“Spin-torque driven ferromagnetic resonance of Co/Ni synthetic layers inspin valves”, Appl. Phys. Lett. 92, 012507 (2008) describe a 1000Angstrom Cu/200 Angstrom Pt/100 Angstrom Cu composite seed layer. Theaforementioned seed layers are not practical with Co/Ni multilayer PMAconfigurations in spintronic devices. Typically, there is a spacerestriction in a direction perpendicular to the planes of the spin valvelayers in advanced devices in order to optimize performance. Seed layersthicker than about 100 Angstroms will require thinning a different layerin the spin valve to maintain a certain minimum thickness for the spinvalve which can easily lead to performance degradation.

In particular, it is highly desirable to design a system whereby a thinseed layer helps to establish PMA in overlying layers, and materialsexhibiting high PMA may be used to induce partial PMA in magnetic layerswith high moments to enable easier FGL oscillation within the highmoment materials and therefore generate a larger oscillating field (Hac)for better MAMR performance.

SUMMARY OF THE INVENTION

One objective of the present invention is to improve the robustness of aspin injection layer (SIL) in a MAMR device.

A second objective of the present invention is to enable an easier FGLoscillation in a MAMR device and thereby produce a larger oscillatingfield (Hac).

According to one embodiment of the present invention, these objectivesare achieved in a bottom SIL structure wherein a stack of layerscomprised of a composite seed layer, [CoFe(t1)/Ni(t2)]_(X) laminatedreference (SIL) layer where x is from about 5 to 50 and Fe contentranges from 0 to 90 atomic %, a non-magnetic spacer, a FeCo FLG, and acapping layer are sequentially formed on a substrate. The seed layerpreferably has a Ta/M1/M2 or Ta/M1 configuration where M1 is an alloysuch as NiCr or a metal having a fcc(111) or (hcp) hexagonal closedpacked (001) crystal orientation such as Ru, and M2 is Cu, Ti, Pd, W,Rh, Au, or Ag. In the case of Pd, Au, and Ag, the M2 layer thickness iskept to a minimum in order to reduce cost and/or minimize any spindepolarization effect. The Ta and M1 layers in the composite seed layerare critical for enhancing the (111) texture in overlying layers. In theSIL, each of the CoFe layers has a thickness (t1) from 0.5 to 5Angstroms and each of the Ni layers has a thickness (t2) of 2 to 10Angstroms. The spacer may be Cu in a CPP-GMR configuration or one ofAlOx, MgO, TiOx, TiAlOx, MgZnOx, ZnOx, or other metal oxides or nitridestypically employed as insulator layers to provide a CPP-TMRconfiguration. The capping layer may be Ru/Ta/Ru, for example.

In another aspect, the bottom SIL structure may be further comprised ofa [CoFe(t1)/Ni(t2)]_(Y) laminate that is exchange coupled with the FeColayer to give a [CoFe(t1)/Ni(t2)]_(Y)/FeCo FGL configuration where y isbetween 5 and 30. Furthermore, one or both of the CoFe/Ni laminates inthe seed/SIL/spacer/FGL/capping layer configuration may be replaced by a[Co(t1)/NiFe(t2)], [Co(t1)/NiCo(t2)], [CoFe(t1)/NiFe(t2)],[CoFe(t1)/NiCo(t2)], [CoFeR(t1)/NiFe(t2)], or [CoFeR(t1)/NiCo(t2)]laminate where R is one of Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg, Mn, orCu. Alternatively, the FeCo FGL layer where Fe content is ≧50 atomic %may be replaced by a CoFe alloy in which Fe content is <50 atomic %.

The present invention also encompasses a top SIL embodiment in which aspin valve comprises a composite seed layer, FeCo FGL, non-magneticspacer, a laminated [CoFe(t1)/Ni(t2)]_(X) SIL, and a capping layersequentially formed on a substrate. The aforementioned layers have thesame composition as mentioned in the bottom SIL embodiments.Alternatively, the FGL may be further comprised of a laminated[CoFe(t1)/Ni(t2)]_(Y) layer to give a [CoFe(t1)/Ni(t2)]_(Y)/FeCo FGLconfiguration. Moreover, the FeCo layer where the Fe content is ≧50atomic % in the preferred top SIL embodiments may be replaced by a CoFealloy in which Fe content is <50 atomic %.

In still another embodiment, the spin valve structure may be representedby seedlayer/[CoFe(t1)/Ni(t2)]_(X)/FeCo/spacer/[CoFe(t1)/Ni(t2)]_(Y)/FeCo/cappinglayer in a seed/SIL/FeCo/spacer/FGL/FeCo/capping layer configuration.This structure provides the maximum benefit of exchange coupling betweena laminated layer with high PMA and a high moment material such as FeCosince easier oscillation is enabled in the FGL simultaneously withimproving the robustness of the SIL. Preferably, the FeCo layer that isexchange coupled with the SIL has a thickness less than the SIL and theFeCo layer exchange coupled with the FGL has a thickness between 100 and300 Angstroms.

In another aspect, the Cu spacer in the aforementioned CPP-GMRconfigurations may be modified by inserting a confining current path(CCP) nano-oxide layer (NOL) between upper and lower portions of the Cuspacer. For example, an amorphous oxide such as AlOx with thin metalpaths therein may be formed between two copper spacer layers in aCu/AlOx/Cu configuration. In a CCP-CPP scheme, the Cu metal path islimited through an insulator template so that the MR ratio in the spinvalve can be enhanced quite significantly.

In all embodiments, the substrate may be a main pole layer and a writeshield may be formed on the capping layer. The spin valve stack may beannealed between 150° C. and 300° C. for a period of 0.5 to 5 hours.Preferably, the FeCo and Ni layers in the (CoFe/Ni)_(X) laminate aredeposited with a very low RF power and a high inert gas pressure tominimize the impinging ion energy so that deposition of a layer does notdamage the CoFe or Ni layer on which it is formed. Thus, the interfacesbetween adjoining CoFe and Ni layers are preserved to maximize the PMAproperty. Furthermore, this method enables the PMA of (CoFe/Ni)_(X) and(CoFe/Ni)_(Y) laminates to be preserved with a substantially thinnerseed layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a MAMR recording head with an acfield assisted perpendicular head design according to a prior artreference.

FIG. 2 shows MH curves for a SIL comprised of a (Co2/Ni6)₂₀ laminate.

FIG. 3 depicts MH curves for a (Co₉₀Fe₁₀2/Ni6)₁₀ laminated SIL with PMAaccording to an embodiment of the present invention.

FIG. 4 depicts MH curves for a (Co₇₅Fe₂₅2/Ni6)₁₀ laminated SIL with PMAaccording to an embodiment of the present invention.

FIG. 5 shows MH curves for a (Co₅₀Fe₅₀2/Ni6)₂₀ laminated SIL with PMAaccording to an embodiment of the present invention.

FIG. 6 shows MH curves for a (Co₃₀Fe₇₀2/Ni6)₁₀ laminated SIL with PMAaccording to an embodiment of the present invention.

FIG. 7 a shows a MAMR structure with a bottom SIL configuration wherethe FGL has a horizontal anisotropy and the SIL has PMA.

FIG. 7 b depicts a MAMR structure according to bottom SIL embodiment ofthe present invention where both of the SIL and FGL have PMA.

FIG. 8 a shows a MAMR structure with a top SIL configuration where theFGL has a horizontal anisotropy and the SIL has PMA.

FIG. 8 b depicts a MAMR structure according to top SIL embodiment of thepresent invention where both of the SIL and FGL have PMA.

FIG. 9 is an MH curve (perpendicular component) of a reference structureincluding a Ta/Ru/Cu seed layer and an overlying FeCo100 FGL layershowing no PMA.

FIG. 10 is a MH curve showing PMA in an exchanged coupled(CoFe/Ni)₁₅/FeCo FGL layer formed on a Ta/Ru/Cu seed according to anembodiment of the present invention.

FIG. 11 is a cross-sectional view of a merged read-write head wherein a“top” STO writer comprised of a main pole, write shield, and spintransfer oscillator structure is formed according to an embodiment ofthe present invention.

FIG. 12 is a cross-sectional view of a merged read-write head wherein a“bottom” STO writer comprised of a main pole, write shield, and spintransfer oscillator structure is formed according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a CPP spin valve structure that includes a(CoFe/Ni)_(X) laminated reference layer (SIL) with perpendicularmagnetic anisotropy that is fully established with a thin composite seedlayer comprised of a lower Ta layer and an upper metal layer withfcc(111) or hcp(001) crystal orientation for enhanced performance inspin transfer oscillators including MAMR devices, STT-MRAM devices, andin other spintronic devices. The free layer or FGL may have a FeCo or(CoFe/Ni)_(Y)/FeCo configuration to enable easier FGL oscillations. Thepresent invention also includes a method of depositing a (CoFe/Ni)_(X)or (CoFe/Ni)_(Y) laminated layer or the like such that the CoFe/Niinterfaces are well preserved and only a thin seed layer is required forestablishing the desired fcc (111) orientation. The terms “field” and“flux” may be used interchangeably when describing FGL components of aMAMR device.

In related U.S. Pat. No. 8,064,244, we disclosed the advantages of Co/Nimultilayer structures having PMA in MRAM applications where the magneticanisotropy of a (Co/Ni)_(X) laminated structure arises from thespin-orbit interactions of the 3d and 4s electrons of Co and Ni atoms.Such interaction causes the existence of an orbital moment which isanisotropic with respect to the crystal axes which are in (111)alignment, and also leads to an alignment of the spin moment with theorbital moment. Since Fe, Co, and Ni atoms have very similar outerelectron configurations, i.e. Fe has [Ar]3d⁶4s² which is one electrondifferent from Co [Ar]3d⁷4s² which has one electron less than Ni[Ar]3d⁸4s², in principle, with some thickness and process optimizations,there should be a possibility of PMA behavior in CoFe/Ni laminatedstructures. In fact, in related U.S. Pat. No. 8,184,411, we describedthe advantages of forming CoFe/Ni laminates having high PMA as a freelayer or reference layer in a spintronic device.

Referring to FIGS. 2-6, MH curves are illustrated for[Co_((100-Z))Fe_(Z)]/Ni laminates where z varies from 0 to 70 atomic %to demonstrate the magnitude (Hc) of the perpendicular (PMA) componentin the films. Note that the distance in the x-axis direction between thevertical portions of the curves in the lower half of each graph isrelated to Hc. MH curves were obtained using a vibrating samplemagnometer (VSM). The upper plot in each figure shows the horizontal toplane component of each magnetic field and the lower plot in each figureillustrates the perpendicular magnetic anisotropy (PMA) component.

In the present invention, we disclose additional spin valve structureswherein CoFe/Ni laminates or the like may be exchange coupled with highmagnetic moment materials such as FeCo to provide a more robust spininjection layer and enable easier oscillations in field generationlayers in MAMR or related spintronic devices. The new structures arebased on our findings that once the number of laminations (x) in a(CoFe/Ni)_(X) structure reaches a large enough number between about 5and 50, there is a sufficient quantity of CoFe and Ni valence electronsto generate a high PMA for spintronics applications from the spin-orbitinteractions. Furthermore, a high PMA layer is able to exchange couplewith an adjacent magnetic layer such as FeCo to impart a certain amountof PMA character in a FeCo layer, for example. In one aspect, acomposite seed layer represented by Ta/M1 where M1 is an upper metallayer having a fcc(111) or hcp(001) crystal orientation such as Ru, Cu,or Au, or an alloy such as NiCr provides an additional advantage ofenhancing the (111) texture in overlying spin valve structures therebyoptimizing the PMA in the laminated SIL and field generation layer.

Referring to FIG. 7 a, a cross-sectional view of a bottom SILconfiguration in a MAMR device according to one embodiment of thepresent invention is shown. Substrate 20 may be a main pole layercomprised of CoFe, NiFe, or CoFeNi, for example. The spin valve stack oflayers 21-25 formed on the substrate is hereafter referred to as a spintransfer oscillator (STO) 60. There is a composite seed layer 21 with afcc(111) lattice formed on the substrate and comprised of a Ta/Ru/Cuconfiguration where a lower Ta layer having a thickness of 5 to 100Angstroms contacts the substrate 20, a middle Ru layer about 10 to 100Angstroms thick is formed on the Ta layer, and an upper Cu layer 1 to100 Angstroms thick is formed on the Ru layer. In another aspect, theupper Cu layer may be removed and a Ta/Ru composite seed layer 21 isemployed wherein the Ta and Ru layers have thicknesses of 5 to 100Angstroms, and 10 to 100 Angstroms, respectively. Optionally, Ru may bereplaced by a metal M1 layer having a fcc(111) or hcp(001) latticestructure. For example, the composite seed layer 21 may have a Ta/Cu orTa/NiCr configuration where Ta thickness is from 5 to 50 Angstroms, Cuthickness is between 20 and 50 Angstroms, and NiCr thickness is from 40to 100 Angstroms.

In another embodiment, the upper Cu layer in the trilayer seed layer 21configuration may be replaced by a metal M2 such as Ti, Pd, W, Rh, Au,Ag, or the like with a thickness for M2 of from 1 to 100 Angstroms togive a Ta/M1/M2 configuration where M1 is unequal to M2. However, it iscritical that the composite seed layer 21 be comprised of a lower Talayer and at least one metal layer having fcc(111) or hcp(001) crystalorientation on the Ta layer to enhance the (111) crystal structure inother layers in the spin valve thereby enlarging the PMA magnitude in anoverlying (CoFe/Ni)_(X) laminated SIL 22. In another embodiment, thecomposite seed layer may comprise NiCr and at least one of Ta and Ru.

Above the composite seed layer 21 is a reference layer or SIL 22 thathas a (A1/A2)_(X) structure where x is between 5 and 50 depending on theMst requirement. Each of the plurality of magnetic A1 layers in theA1/A2 laminate has a thickness from 0.5 to 5 Angstroms, and preferablybetween 1.5 to 3 Angstroms. Each of the plurality of magnetic A2 layersin the SIL has a thickness from 2 to 10 Angstroms, and preferablybetween 3.5 and 8 Angstroms. Preferably, the thickness t2 of an A2 layeris greater than an A1 layer thickness t1, and more preferably, t2˜2×t1in order to optimize the spin orbit interactions between adjacent A1 andA2 layers. In addition, A1 and A2 layers are deposited by a method thatpreserves the A1/A2 interfaces as described in a later section. In oneaspect, when t1 is less than or equal to about 2 Angstroms, the A1 layermay be considered as a “close-packed” layer and not necessarily having a(111) crystal orientation. In one embodiment, the A1 layer is comprisedof CoFe and the A2 layer is Ni, and each of the CoFe layers in the(CoFe/Ni)_(X) laminate has a [Co_((100-Z))Fe_(Z)] composition in which zis from 0 to 90 atomic %.

In an alternative embodiment, the (CoFe/Ni)_(X) laminate in SIL 22 maybe replaced by one of [Co(t1)/NiFe(t2)]_(X), [Co(t1)/NiCo(t2)]_(X),[CoFe(t1)/NiFe(t2)]_(X) or [CoFe(t1)/NiCo(t2)]_(X) wherein the Nicontent in the NiFe and NiCo layers ranges from 50 to 100 atomic %.

The present invention also encompasses an embodiment wherein thelaminated SIL 22 is comprised of [CoFeR(t1)/Ni(t2)]_(X),[CoFeR(t1)/NiFe(t2)]_(X), or [CoFeR(t1)/NiCo(t2)]_(X) where R is a metalsuch as Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg, Mn, or Cu. Preferably, the Rcontent in the CoFeR alloy is less than 10 atomic % and a CoFeR layerhas a t1 thickness.

Above the SIL 22 is a non-magnetic spacer 23 that may be comprised of Cuin a CPP-GMR configuration, or a dielectric layer such as AlOx, MgO,TiOx, TiAlOx, MgZnOx, ZnOx, or other metal oxides or metal nitridestypically employed as insulator layers to give a CPP-TMR configuration.MgO is especially preferred as a non-magnetic spacer in a CPP-TMRconfiguration because a higher MR ratio is achieved than with othermetal oxides. The metal oxide may be formed by first depositing themetal preferably by a sputter deposition method and then performing aradical oxidation (ROX) or natural oxidation (NOX) process. A secondmetal layer may be deposited on the resulting oxidized metal layer tocomplete the non-magnetic spacer process. A Cu spacer 23 may have athickness from 15 to 150 Angstroms, and preferably between 20 to 60Angstroms. Preferably, the metallic spacer 23 is sufficiently thick toprevent coupling between the SIL 22 and FGL 24. Moreover, a Cu spacer isselected because of having excellent conductivity to enable a current topass through the STO layers 21-25 in a current perpendicular to plane(CPP) direction during a read or write process.

FGL 24 is formed on the non-magnetic spacer 23 and preferably has a highspin polarization and a small magnetic damping coefficient in order toenable spin transfer magnetization switching in the spintronic device.FGL is a magnetic (ferromagnetic) layer made of FeCo or an alloy thereofcontaining at least one atom selected from Al, Ge, Si, Ga, B, C, Se, andSn and has a large magnetic moment (high Bs) aligned along an easy axisdirection that is switched to an opposite direction when a spin torqueof sufficient magnitude is applied. In a preferred embodiment, FGL 24 isa FeCo layer with a Fe content ≧50 atomic % and a thickness from 50 to300 Angstroms. However, the FGL may also be a CoFe layer in which Fecontent is <50 atomic %.

The uppermost layer in STO 60 is a composite capping layer 25 thatcontacts a write shield 26 according to one embodiment of the presentinvention. In one aspect, the capping layer 25 has a Ru/Ta/Ruconfiguration where the upper Ru layer is used to provide oxidationresistance and serves as an excellent electrical contact. A substantialreduction in critical current density (Jc) occurs when a thin Ru layeris employed as a capping layer in a STT-MRAM embodiment due to thestrong spin scattering effect of Ru. Critical current density (Jc) ispreferably about 10⁶ A/cm² to be viable for spin-transfer magnetizationswitching in the 90 nm technology node and beyond. Higher values coulddestroy a thin tunnel barrier made of AlOx, MgO, or the like as employedin a CPP-TMR embodiment of the present invention. The Ta layer may beincluded to offer etch resistance in subsequent processing steps.Optionally, other capping layer materials used in the art may beemployed as capping layer 25.

It should be understood that to achieve a desirable MAMR device, a largeHac must be generated by the FGL which means a high Bs in the FGLmaterial is required since Hac increases as Bs becomes larger. However,once the Bs becomes too large, the critical current density is too largeand will raise a serious reliability concern. According to Slonczewski'smodel in the reference cited earlier, once the FGL has PMA or partialPMA, the critical current density for spin transfer could be greatlyreduced. Therefore, we were motivated as described hereinafter to employPMA in a CoFe/Ni laminate, for example, to induce partial PMA in a highBs FGL such as FeCo through exchange coupling. In theory, a compositeFGL having a laminate with high PMA and a high Bs material should resultin a high moment and partial PMA to greatly assist FGL oscillations andincrease Hac.

Referring to FIG. 8 a, the STO structure in FIG. 7 a is modified toinclude a laminate in the field generation layer to give a preferredembodiment with a FGL 28 having an (A1/A2)_(Y)/FeCo or[CoFe(t1)/Ni(t2)]_(Y)/FeCo configuration where y is between 5 and 30.Furthermore, one or both of the (A1/A2)_(X) and (A1/A2)_(Y) laminates inthe resulting seed/SIL/spacer/FGL/capping layer configuration may bereplaced by a laminate where (A1/A2) is one of [Co(t1)/NiFe(t2)],[Co(t1)/NiCo(t2)], [CoFe(t1)/NiFe(t2)], [CoFe(t1)/NiCo(t2)],[CoFeR(t1)/Ni(t2)]_(X), [CoFeR(t1)/NiFe(t2)], or [CoFeR(t1)/NiCo(t2)].Alternatively, the FeCo high Bs layer with a thickness of 50 to 300Angstroms in the composite FGL may be replaced by a FeCo alloy with anatom selected from Al, Ge, Si, Ga, B, C, Se, and Sn, or by a CoFe alloywhere Fe content is <50 atomic %.

Referring to FIG. 7 b, a top SIL embodiment of the present invention isdepicted which comprises the same layers as in FIG. 7 a except the SIL22 and FGL 24 have switched positions such that the STO 60 formed on thesubstrate 20 is represented by seed layer/FGL/non-magneticspacer/SIL/capping layer where the seed layer contacts the substrate andthe capping layer is the uppermost layer. For a CPP-GMR embodiment, STO60 may have a Ta/Ru/Cu/FeCo/Cu/[CoFe(t1)/Ni(t2)]_(Y)/Ru/Ta/Ruconfiguration. Alternatively, the Ta/Ru/Cu seed layer, FeCo FGL, CoFe/Nilaminate (SIL), and capping layer may be replaced by other suitablematerials as indicated previously for the bottom SIL embodiments.Moreover, the Cu spacer may be replaced by a dielectric layer such asAlOx, MgO, TiOx, TiAlOx, MgZnOx, ZnOx, or other metal oxides or metalnitrides employed as insulator layers to give a CPP-TMR configuration.In one aspect, the substrate 20 may be a main pole layer which functionsas a bottom electrode and a write shield 26 may be formed on the cappinglayer to serve as a top electrode in the spintronic device.

Referring to FIG. 8 b, the top SIL embodiment in FIG. 7 b is modified toinclude a laminate in the field generation layer to give a (A1/A2)_(Y)or [CoFe(t1)/Ni(t2)]_(Y)/FeCo FGL configuration where y is between 5 and30. Furthermore, one or both of the (A1/A2)_(X) and (A1/A2)_(Y)laminates in the resulting seed/FGL/spacer/SIL/capping layer STO 60 maybe replaced by a laminate where (A1/A2) is one of [Co(t1)/NiFe(t2)],[Co(t1)/NiCo(t2)], [CoFe(t1)/NiFe(t2)], [CoFe(t1)/NiCo(t2)],[CoFeR(t1)/Ni(t2)]_(X), [CoFeR(t1)/NiFe(t2)], or [CoFeR(t1)/NiCo(t2)].Alternatively, the FeCo high Bs layer in the composite FGL may bereplaced by a FeCo alloy containing at least one atom selected from Al,Ge, Si, Ga, B, C, Se, and Sn, or by a CoFe alloy in which Fe content is<50 atomic %.

In yet another embodiment, both of the SIL 22 and FGL 28 may becomprised of a composite including a FeCo or alloy layer with a high Bs,and an (CoFe/Ni) laminate or the like that exchange couples with thehigh Bs layer to give an (A1/A2)_(X)/FeCo or (A1/A2)_(Y)/FeCoconfiguration, respectively, to generate partial PMA in the FeCo oralloy layer. For instance, the STO 60 on substrate 20 may have a CPP-GMRconfiguration represented byTa/Ru/Cu/[CoFe(t1)/Ni(t2)]_(X)/FeCo/Cu/[CoFe(t1)/Ni(t2)]_(Y)/FeCo/Ru/Ta/RuorTa/Ru/Cu/[CoFe(t1)/Ni(t2)]_(Y)/FeCo/Cu/[CoFe(t1)/Ni(t2)]_(X)/FeCo/Ru/Ta/Ru.In this case, the FeCo layer coupled with the SIL preferably has athickness of about 50 to 100 Angstroms and less than that of the SIL. Asindicated earlier, a dielectric layer such as AlOx, MgO, TiOx, TiAlOx,MgZnOx, ZnOx, or other metal oxides or metal nitrides employed asinsulator layers may be used instead of a Cu spacer to provide a CPP-TMRconfiguration. Furthermore, the Ta/Ru/Cu seed layer, one or both of theFeCo layers in the FGL and SIL, one or both of the CoFe/Ni laminates,and the capping layer may be replaced by another suitable material asdescribed previously.

Referring to FIG. 9, a MH curve is illustrated for a reference structureincluding a 100 Angstrom thick FeCo layer grown on a Ta10/Ru20/Cu20 seedlayer. The results show no PMA established in the FeCo layer as expectedfrom a magnetic material that typically has a magnetic moment alignedlongitudinally along an easy axis direction parallel to the planes ofthe layers in a spin valve stack.

On the other hand, when a composite (CoFe2/Ni5)₁₀/FeCo100 FGL is formedon the Ta10/Ru20/Cu20 seed layer according to an embodiment of thepresent invention, the resulting MH curve in FIG. 10 shows that asubstantial PMA of around 1600 Oe is generated by exchange couplingbetween the (CoFe/Ni)_(Y) laminate and the FeCo high Bs layer. In bothFIG. 9 and FIG. 10, Fe content in the FeCo100 layer is 70 atomic % andannealing was performed at 220° C. for 2 hours. The other (A1/A2)_(Y)laminates described herein are expected to give similar PMA behavior ina composite [(A1/A2)_(Y)/FeCo100] FGL.

The present invention also anticipates that in a CPP-GMR configurationhaving either a bottom SIL or top SIL structure, the Cu spacer may bereplaced by a confining current path (CCP) CPP GMR sensor where thecurrent through the Cu spacer is limited by the means of segregatingmetal path and oxide formation. With a CCP-CPP scheme, the Cu metal pathis limited through an insulator template or nano-oxide layer (NOL) sothat the MR ratio can be significantly enhanced. An NOL layer (notshown) may be formed by a well known method involving deposition of anAlCu layer on a lower Cu layer followed by a pre-ion treatment (PIT) andan ion-assisted oxidation (IAO) process to convert the AlCu layer intoan AlOx matrix having segregated Cu pathways (current confining paths)therein. Thereafter, an upper Cu layer is deposited on the NOL (CCP)layer.

In all STO embodiments described herein, a key feature is that thecomposite seed layer 21 having a Ta/M1 or Ta/M1/M2 configurationenhances the (111) lattice structure and PMA in laminated SIL 22 in abottom SIL configuration, or in a laminated FGL 28 in a bottom FGLconfiguration. Moreover, laminated SIL 22 and laminated FGL 28 aredeposited in a manner that preserves the CoFe/Ni (A1/A2)_(X) or(A1/A2)_(Y) interfaces formed therein.

Referring to FIG. 11, the MAMR structure or spin transfer oscillator(STO) 60 may be formed in a write head 80. In the exemplary embodiment,the STO writer 80 is pictured as part of a merged read-write head wherethe read head 75 includes top and bottom shields 74 a, 74 b, and asensor 73 between the aforementioned shields. STO writer 80 is comprisedof a main pole 76, a trailing shield 77 and a wire 78 for injectingcurrent into the spin transfer oscillator structure 60 which is shown ina “top FGL/bottom SIL” STO configuration. As mentioned earlier, a bottomSIL configuration preferably has a (CoFe/Ni)_(X) laminate or the likeexchanged coupled with a high Bs layer such as FeCo and the SIL 22 isseparated from the main pole layer 76 by a seed layer. The (CoFe/Ni)_(X)laminate has a PMA aligned in the same direction as the media movingdirection. Note that the FGL or oscillator layer is formed closer to thefirst electrode (trailing shield 77) than the SIL or reference layer andhas a magnetization direction which is free to rotate as indicated bythe layer with two arrows and a dotted circle in STO 60.

In an alternative embodiment as depicted in FIG. 12, the positions ofthe field generation (oscillator) layer and SIL may be switched to givea “bottom FGL/top SIL” STO 60 configuration. In this case, the FGL isseparated from the main pole 76 by a seed layer and a capping layerseparates the SIL from the trailing shield 77. The merged read-writehead moves in the direction indicated while suspended on an air bearingsurface above substrate 71 having media tracks 72 formed thereon.

With regard to a process of forming the various spin valve structures ofthe aforementioned embodiments, all of the layers in the CPP spin valvestack may be laid down in a sputter deposition system. For instance, theCPP stack of layers may be formed in an Anelva C-71.00 thin filmsputtering system or the like which typically includes three physicalvapor deposition (PVD) chambers each having 5 targets, an oxidationchamber, and a sputter etching chamber. At least one of the PVD chambersis capable of co-sputtering. Typically, the sputter deposition processinvolves an argon sputter gas with ultra-high vacuum and the targets aremade of metal or alloys to be deposited on a substrate. All of the CPPlayers may be formed after a single pump down of the sputter system toenhance throughput.

The present invention also encompasses an annealing step after alllayers in the CPP spin valve structure have been deposited. The STO 60may be annealed by applying a temperature between 150° C. and 300° C.,and preferably between 180° C. and 250° C. for a period of 0.5 to 5hours. No applied magnetic field is necessary during the annealing stepbecause PMA is established due to the (111) texture in the compositeseed layer 21 and due to the CoFe—Ni spin orbital interactions in thelaminated SIL 22, and in laminated FGL 28. However, the presentinvention also anticipates that a field may be applied during annealingto further increase PMA in the spin valve (STO) structure.

An important feature of the present invention is the method fordepositing a (A1/A2)_(X) laminated SIL 22 and a laminated (A1/A2)_(Y)FGL 28. It should be understood that the same deposition process appliesto other laminates described herein such as (Co/NiFe)_(X),(Co/NiCo)_(X), (CoFe/NiFe)_(X) or (CoFe/NiCo)_(X), (CoFeR/Ni)_(X),(CoFeR/NiFe)_(X), and (CoFeR/NiCo)_(X). In particular, a lower RF powerand high Ar pressure are utilized to avoid damaging the substrate onwhich each CoFe or Ni layer is deposited in order to preserve theresulting CoFe/Ni interfaces and enhance the PMA property therein. Inother words, the ion energy impinging on recently deposited CoFe and Nisurfaces is minimized during sputter deposition of subsequent CoFe andNi layers to reduce damage from ion bombardment during the sputteringprocess. In one embodiment, each of the A1 and A2 layers in a laminatedlayer 22, 28 is laid down in a DC magnetron sputter deposition chamberby a process comprising a RF power of less than 200 Watts, and an Arflow rate of >15 standard cubic centimeters per minute (sccm).Deposition of each A1 and A2 layer requires less than a minute and totaltime necessary to form a (A1/A2)₂₀ structure is less than about an hour.

Once all the layers in the STO 60 are formed, the STO is patterned intoa rectangular, oval, circular, or other shapes from a top-downperspective along the media moving direction by a well known photoresistpatterning and reactive ion etch transfer sequence. Thereafter, aninsulation layer (not shown) may be deposited on the substrate 20followed by a planarization step to make the insulation layer coplanarwith the capping layer 25. Next, the trailing shield 77 may be formed onthe STO 60 and insulation layer as appreciated by those skilled in theart.

Example 1

A series of STO structures comprising a bottom SIL configuration wasfabricated to provide examples of the first embodiment. The bottom SILconfiguration is represented byTa10/Ru20/Cu20/[Co_((100-Z))Fe_(Z)2/Ni5]_(X)/spacer/FeCo100/Ru10/Ta40/Ru30where the number following each layer is the thickness in Angstroms.Ta/Ru/Cu is employed as the seed layer, FeCo100 is the FGL, and a(CoFe2/Ni5)_(X) laminate is the SIL in which each CoFe layer is 2Angstroms thick and each Ni layer is 5 Angstroms thick and x ismaintained between 5 and 50. Fe content in the CoFe laminated layers iskept between 0 and 90 atomic %. A Cu or another metallic spacer isemployed for CPP-GMR applications while a AlOx, MgO, TiOx, TiAlOx,MgZnOx, or ZnOx spacer is used for CPP-TMR structures. The capping layeris a Ru10/Ta40/Ru30 composite. Based on torque measurements, we deducedthat Hk for each (CoFe/Ni)_(X) stack is >15000 Oersted (Oe).

Example 2

A preferred bottom SIL configuration was fabricated and is representedbyTa10/Ru20/Cu20/[Co_((100-Z))Fe_(Z)2/Ni5]_(X)/spacer/[Co_((100-Z))Fe_(Z)2/Ni5]_(Y)/FeCo100/Ru10/Ta40/Ru30.This structure is a modification of the first embodiment where a(CoFe/Ni)_(Y) laminate is inserted between the non-metallic spacer andthe FeCo100 layer to give a composite FGL where y is between 5 and 30.Since the [Co_((100-Z))Fe_(Z)2/Ni5]_(Y) laminate has a strong magneticcoupling with the FeCo100 layer, the large PMA of the[Co_((100-Z))Fe_(Z)2/Ni5]_(Y) laminate will force the anisotropy of theFeCo to tilt partially toward the perpendicular to plane direction sothat the entire FGL can easily oscillate under a low current density.

Example 3

A series of STO structures comprising a top SIL configuration wasfabricated according to another embodiment of the present invention. Thetop SIL configuration is represented byTa10/Ru20/Cu20/FeCo100/spacer/[Co_((100-Z))Fe_(Z)2/Ni5]_(X)/Ru10/Ta40/Ru30where the number following each layer is the thickness in Angstroms.Ta/Ru/Cu is employed as the seed layer, FeCo100 is the FGL, and a(CoFe2/Ni5)_(X) laminate is the SIL in which each CoFe layer is 2Angstroms thick and each Ni layer is 5 Angstroms thick and x ismaintained between 5 and 50. Fe content in the CoFe laminated layers iskept between 0 and 90 atomic %. Cu or another metallic spacer isemployed for CPP-GMR embodiments while a AlOx, MgO, TiOx, TiAlOx,MgZnOx, or ZnOx spacer is used for CPP-TMR structures. The capping layeris a Ru10/Ta40/Ru30 composite. Based on torque measurements, we deducedthat Hk for each (CoFe/Ni)_(X) stack is >15000 Oersted (Oe).

Example 4

A preferred top SIL configuration was fabricated and is represented byTa10/Ru20/Cu20/[Co_((100-Z))Fe_(Z)2/Ni5]_(Y)/FeCo100/spacer/[Co_((100-Z))Fe_(Z)2/Ni5]_(X)/Ru10/Ta40/Ru30. This structure is a modification of the top SIL embodimentin Example 3 where a (CoFe/Ni)_(Y) laminate is inserted between the seedlayer and the FeCo100 layer to give a composite FGL where y is between 5and 30. Since the [Co_((100-Z))Fe_(Z)2/Ni5]_(Y) laminate has a strongmagnetic coupling with the FeCo100 layer, the large PMA of the[Co_((100-Z))Fe_(Z)2/Ni5]_(Y) laminate will force the anisotropy of theFeCo to tilt partially toward the perpendicular to plane direction sothat the entire FGL can easily oscillate under a low current density.

Example 5

According to another preferred embodiment of the present invention, thetop SIL configuration in Example 4 may be further modified to include aFeCo layer coupled with the [Co_((100-Z))Fe_(Z)2/Ni5]_(X) SIL to yield acomposite SIL and a structure represented byTa10/Ru20/Cu20/[Co_((100-Z))Fe_(Z)2/Ni5]_(Y)/FeCo/spacer/[Co_((100-Z))Fe_(Z)2/Ni5]_(X)/FeCo/Ru10/Ta40/Ru30.In this example, the lower FeCo layer coupled to the laminated FGL has athickness between 50 and 300 Angstroms while the upper FeCo layercoupled to the SIL has a thickness between 50 and 100 Angstroms. Thecomposite SIL configuration is employed to strengthen SIL robustness(stability) and the composite FGL is used as described previously toassist the FeCo FGL oscillations. In other words, exchange couplingbetween a high Bs layer such as FeCo and a PMA laminate like(FeCo/Ni)_(X) or the like may be advantageously used to maintain themagnetization direction of the SIL as a reference layer in a STO device.The spacer may be Cu or a metal oxide as indicated in previous examples.

We have described various embodiments of bottom SIL and top SILconfigurations in a STO device wherein magnetic coupling between alaminate with high PMA and a high Bs layer is employed to partially tiltthe anisotropy of the high Bs layer toward a perpendicular to planedirection thereby enabling an easier FGL oscillation under low currentdensity and produce a larger oscillating field (Hac) for betterperformance. In addition, a composite SIL involving magnetic couplingbetween a laminate with large PMA and a high Bs layer can be used tostrengthen the robustness of the spin injection layer for improvedperformance. A high PMA in the laminates is achieved by a depositionmethod for CoFe and Ni films that preserves the CoFe/Ni interfaces andthereby maintains PMA therein to provide improved performance even witha thin seed layer.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A spin transfer oscillator (STO) structure in a spintronic device, comprising: (a) a composite seed layer comprising at least a lower Ta layer formed on a substrate and a metal (M1) layer having a fcc(111) or hcp(001) crystal structure contacting the lower Ta layer; (b) a field generation layer (FGL) that has an (A1/A2)_(Y)/FeCo alloy configuration where A1 is one of Co, CoFe, or CoFeR wherein R is one of Ru, Rh, Pd, Ti, Zr, Hf, Ni, Cr, Mg, Mn, or Cu, and A2 is one of Ni, NiCo, and NiFe, and Y is a number of laminations, the FeCo alloy includes one or more of Al, Ge, Si, Ga, C, Se, and Sn and contacts a bottom surface of a non-magnetic spacer; (c) the non-magnetic spacer; (d) a laminated spin injection layer (SIL) with high perpendicular magnetic anisotropy (PMA) and with a (A1/A2)_(X) configuration contacting a top surface of the non-magnetic spacer wherein x is in a range from 5 to 50, and a thickness (t2) of each A2 magnetic layer is greater than a thickness (t1) of each A1 magnetic layer; and (e) a capping layer contacting a top surface of the SIL.
 2. The STO structure of claim 1 wherein the Fe content in the FeCo alloy is less than 50 atomic %.
 3. The STO structure of claim 1 wherein the (A1/A2)_(Y) laminate contacts the seed layer.
 4. The STO structure of claim 3 where Y is from about 5 to
 30. 